Producing hydrocarbons from an underground reservoir requires those fluids to be driven to the producing wells, and then lifted several hundred meters against the force of gravity. The large-scale behavior of a reservoir can be described by considering the drive energy of the reservoir and its surroundings. The producing lifetime of a reservoir may generally be categorized as follows:    Primary recovery: where the natural drive energy locked up in the reservoir and its surroundings is used to produce hydrocarbons    Secondary recovery: where the natural drive energy of the reservoir is supplemented by injection of a fluid, normally water or gas    Tertiary recovery: where residual hydrocarbons trapped after conventional secondary recovery techniques are mobilized by the injection of fluids that are not normally found in the reservoir (e.g. surfactants, steam, and polymers)
Enhanced oil recovery (EOR) involves methods of recovering more oil from a reservoir than can be obtained from the naturally occurring drive mechanisms such as solution gas drive (fluid expansion) or water influx. EOR involves the introduction of artificial/supplemental forces or energy into the reservoir for the purpose of aiding the natural drive mechanisms. EOR can occur at any stage in the production life, although it is usually relegated to secondary or tertiary aspects. Some types of EOR include water flooding, gas flooding, steam injection, and carbon dioxide injection.
Planning an EOR project demands meticulous attention to the various factors that influence the selection of an EOR candidate. Although EOR is a powerful technique for recovering more hydrocarbons from a producing reservoir, it is not always a commercially viable option. Traditionally the EOR potential of candidate reservoirs is evaluated using classical reservoir engineering techniques. Engineers quantify EOR potential one field at a time using numerical methods and field specific data. This process can be very time-consuming and often yields inaccurate or incomplete results. For purposes of this application, “gas flooding” refers to gas injected to access oil not accessible to a waterflood. In a gas flooding operation, “injected gas” refers to the gas injected. “Injectant” refers to an enriching agent such as propane, butane, hydrogen sulfide, or other substances added to the gas injected to improve recovery.